1. Field of the Invention
The present invention relates to a microchip fuse suitable for surface mounting on a printed board or the like.
2. Description of Prior Art
A microchip fuse according to the prior art comprises a cylindrical casing, an elongated fusible element extending through a hollow portion between the opposite ends of the casing, and cap-shaped conductive terminals fitted to the opposite ends of the casing and electrically connected to the ends of the fusible element (for example, refer to U.S. Pat. No. 4,920,327).
A fuse is restricted in terms of the distance between the tip ends of the opposite conductive terminals, that is, the creepage distance must be kept at a required length depending on the required electric characteristics in order to attain desired electric characteristics. According to the microchip fuse of such prior art as above, since the circumferential portions of the cap-shaped conductive terminals are fitted to the casing in such a manner as to cover the side surface of the casing, the distance between the tip ends of the conductive terminals will be shorter than the distance between the opposite end surfaces of the casing by an amount equal to twice the width of the circumferential portions of the conductive terminals. As a consequence, the total length of the fuse in the direction of the conductive terminals has been limited to approximately 6 mm due to the limitation imposed by the creepage distance mentioned above.
On the other hand, there have been frequently used electrodes having a metallic sheet in an L-shape and attached to the opposite end surfaces and their adjacent circumferential surface of a cylindrical casing. However, as the portions of the metallic sheet extend over the side surfaces of the casing, there has also been a limitation regarding shortening the length of the fuse between the electrodes for the same reason as the cap-shaped conductive terminals as explained above.
Notwithstanding the above-mentioned limitations, since miniaturization of electronic appliances proceeds increasingly in the recent years, there have been demands made for miniaturization of electronic components. Besides, demands for quick acting microchip fuses are also so considerable that for this purpose it has become necessary to further shorten the length of the fusible element. There are now required fuses wherein the total length of the fuse between the electrodes is shorter than 6 mm. In fact, lengths in the order of 1.5 mm are desired. For a microchip fuse satisfying such a requirement since a conductive terminal construction and metallic sheet electrodes according to the prior art require the width of the circumferential portion to be in the order of 0.5-1 mm, they are unsuitable for use in highly miniaturized fuses.
It is conceivable to construct a thin electrode by applying metallic vapor deposition or the like to the opposite end surfaces of a casing. However, metallic vapor deposition requires a vacuum apparatus, resulting in expensive facilities and targets. Furthermore, production efficiency is not good, due to batch-wise production, and production costs increase. Consequently, this method is not actually applied.
As explained above, the casing for the microchip fuse according to a prior art is cylindrical in many cases. The sectional dimension of the microchip fuse having a total length of about 6 mm is normally in the order of 2-3 mm, so that it is not easy from the production point of view to extend such a very thin fusible element having a size of about some ten .mu.m between the opposite end surfaces of the casing through a small cylindrical hollow portion.